Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y or θ-plane.
The frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0. In order to maximize the accuracy of NMR measurements, the resonant frequency of the excitation/detection circuitry is set to be equal to the frequency of interest. The resonant frequency of the excitation/detection circuitry is generallyv=1/√{square root over (LC)}  [1]where L and C are the effective inductance and capacitance, respectively, of the excitation/detection circuitry.
Additionally, in order to maximize the transfer of RF energy into the RF coils, the impedance of each coil is matched to the impedance of the transmission line and associated components used to couple RF energy into the coil. If the coil is not impedance-matched, a sub-optimal fraction of the RF energy sent to the coil actually enters the coil. The rest of the energy is reflected out, and does not contribute to the NMR measurements.
Variable capacitors may be used to adjust the circuit resonant frequency and to ensure optimal impedance matching. Typical variable capacitors used in NMR applications are non-magnetic capacitors capable of operating at voltages on the order of several kV. Such variable capacitors are often placed in a space-constrained region within the nuclear magnetic resonance probe, for example in a region immediately underneath the NMR sample coil. The tight spaces available within typical NMR probes and the high voltages applied to such variable capacitors may lead to undesirable arcing from the capacitors to surrounding probe components held at lower voltages. In addition, some NMR circuits may suffer from undesirable stray capacitance, which may degrade the circuits' performance.